Action-based Dynamical Modeling for the Milky Way Disk: The Influence of Spiral Arms
RoadMapping is a dynamical modeling machinery developed to constrain the Milky Way's (MW) gravitational potential by simultaneously fitting an axisymmetric parametrized potential and an action-based orbit distribution function (DF) to discrete 6D phase-space measurements of stars in the Galactic disk. In this work we demonstrate RoadMapping's robustness in the presence of spiral arms by modeling data drawn from an N-body simulation snapshot of a disk-dominated galaxy of MW mass with strong spiral arms (but no bar), exploring survey volumes with radii 500pc<=r_max<=5kpc. The potential constraints are very robust, even though we use a simple action-based DF, the quasi-isothermal DF (qDF). The best-fit RoadMapping model always recovers the correct gravitational forces where most of the stars that entered the analysis are located, even for small volumes. For data from large survey volumes, RoadMapping finds axisymmetric models that average well over the spiral arms. Unsurprisingly, the models are slightly biased by the excess of stars in the spiral arms. Gravitational potential models derived from survey volumes with at least r_max=3kpc can be reliably extrapolated to larger volumes. However, a large radial survey extent, r_max~5kpc, is needed to correctly recover the halo scale length. In general, the recovery and extrapolability of potentials inferred from data sets which were drawn from inter-arm regions appear to be better than those of data sets drawn from spiral arms. Our analysis implies that building axisymmetric models for the Galaxy with upcoming Gaia data will lead to sensible and robust approximations of the MW's potential.